Dietary fruits and vegetables and cardiovascular diseases risk

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Alissa, Eman M and Ferns, Gordon A (2015) Dietary fruits and vegetables and cardiovascular diseases risk. Critical Reviews in Food Science and Nutrition, 57 (9). pp. 1950-1962. ISSN 1040-8398

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Dietary Fruits and Vegetables and Cardiovascular Diseases Risk

Eman M Alissa(1), Gordon A Ferns(2)

(1) Faculty of Medicine, King AbdulAziz University, P.O. Box 12713, Jeddah 21483, Saudi

Arabia

(2) Medical Education and Metabolic Medicine, Brighton and Sussex Medical School, University

of Brighton, BN1 9PH, UK

Address for correspondence:

Eman M Alissa

Faculty of Medicine,

King Abdul Aziz University,

PO Box 12713

Jeddah 21483,

Kingdom of Saudi Arabia

Tel: (966) 2 6400000 Ext. 23432

Fax: (966) 2 6643499

e-mail: em_alissa@yahoo.com

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Abstract

Diet is likely to be an important determinant of cardiovascular disease (CVD) risk. In this

article, we will review the evidence linking the consumption of fruit and vegetables and

CVD risk.

Efforts were initially focused on individual protective nutrients, such as vitamin E,

vitamin C, and β-carotene, in an attempt to identify putative intervention strategies.

However these have generally proven to be disappointing when tested in clinical trials.

The evidence now suggests that a complicated set of several nutrients may interact with

genetic factors to influence CVD risk. Therefore, it may be more important to focus on

whole foods and dietary patterns rather than individual nutrients to successfully impact

on CVD risk reduction.

The initial evidence that fruit and vegetable consumption has a protective effect against

CVD came from observational studies. However uncertainty remains about the

magnitude of the benefit of fruit and vegetable intake on the occurrence of CVD and

whether the optimal intake is five portions or greater. Results from randomized controlled

trials do not show conclusively that fruit and vegetable intake protects against CVD, in

part because the dietary interventions have been of limited intensity to enable optimal

analysis of their putative effects.

The protective mechanisms of fruit and vegetables may include some of the known

bioactive nutrient effects dependent on their antioxidant, anti-inflammatory and

electrolyte properties, but also include their functional properties, such as low glycemic

load and energy density. Taken together, the totality of the evidence accumulated so far

does appear to support the notion that increased intake of fruits and vegetables may

reduce cardiovascular risk. It is clear that fruit and vegetables should be eaten as part of a

balanced diet, as a source of vitamins, fiber, minerals and phytochemicals.

A clearer understanding of the relationship between fruit and vegetable intake and

cardiovascular risk would provide health professionals with significant information in

terms of public health and clinical practice.

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Introduction

Dietary factors play an important role in the development and progression of

cardiovascular disease (CVD) (Din,2002, Hu & Willett, 2002), and dietary optimization

must be considered an important lifestyle intervention for the management of existing

CVD and its prevention (Ferrari et al.,2004).

The importance of diet in maintaining optimal health has emerged from several lines of

evidence including observations that the rates of particular diseases differ significantly

between cultures; with dietary variation being an obvious factor contributing to these

differences (Key et al.,1996, Ness & Powles,1997). In contrast to the literature on cancer

risk (Steinmetz & Potter,1991), most studies of diet and CVD have reported associations

of risk with specific nutrients rather than with entire foods (Willett,1993). Nonetheless,

several studies have reported on the role of overall dietary patterns in predicting

cardiovascular risk (Hu,2002). In these studies, high intakes of fruits, vegetables,

legumes, whole grains, poultry, and fish were associated with lower cardiovascular risk

whereas a western pattern diet, characterized by high intakes of red and processed meat,

sweets, fried food, and refined grains were associated with higher cardiovascular risk

independent of lifestyle factors (Hu et al.,2000, Fung et al.,2001). The utility of adopting

a whole diet approach (e.g., the Dietary Approaches to Stop Hypertension (DASH),

Mediterranean, and Portfolio diets), with emphasis on antioxidant-rich fruits and

vegetables and whole grains, in CVD prevention has been widely supported (De Waart et

al.,2001; Sacks et al.,2001; Trichopoulou et al.,2003; Wiztum,1994).

Diets rich in antioxidants derived from fruits and vegetables are thought to be protective

against reactive oxygen species, and this, provides a potential mechanism through which

they may prevent cancer and CVD. This has, in part, been the rationale for the WHO and

other health authorities to recommend the consumption of these foods (Committee on

Diet and Health, National Research Council,1989, WHO,2003). Dietary advice for CVD

prevention is complex for a number of reasons. Fruit and vegetables differ qualitatively

and quantitatively in their content of specific antioxidants; particularly the relative

balance between these antioxidants and their bioavailability. Furthermore it is unclear

what the optimal number of portions should be consumed per day and how many portions

should be taken as fresh, processed, or dried fruit. It is also unclear whether the benefits

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of a diet rich in fruit and vegetables would be more evident in primary versus secondary

prevention (WHO,2003). A clear understanding of the relationship between fruit and

vegetable intake and cardiovascular risk would provide health professionals with

significant information in terms of public health and clinical practice (Steptoe et

al.,2003). The American Heart Association (AHA) has had a longstanding commitment

to provide information about the role of nutrition in CVD risk reduction (Krauss et

al.,2000). Nutritional advice is often easier to understand in the context of foods rather

than the individual nutrients they contain. The AHA previously recommended a diet that

contains more than 5 servings of fruit and vegetables daily (Kromhout,2001).

Nevertheless, there remains a low intake of fruits and vegetables worldwide (Hall et

al.,2009). An ecological study examined associations between fruits and vegetables

consumption in the population and six health outcomes (ischemic heart diseases, stroke

and cancers of the stomach, oesophagus, colon, rectum and lung) using global data. Data

were obtained from national representative dietary surveys and the Food and Agriculture

Organization of the United Nations, with analyses being stratified by 14 geographical

regions. The analysis suggested that 2.6 million deaths worldwide and 31% of coronary

heart disease and 19% of ischaemic stroke may be due to a suboptimal consumption of

fruits and vegetables (Lock et al.,2005). More recently, dietary guidelines for Americans

recommend that most people should eat at least 9 servings (41/2 cups) of fruits and

vegetables a day, 4 servings (2 cups) of fruits, and 5 servings (2.5 cups) of vegetables,

based on a 2000 kcal diet (United States Department of Agriculture 2010).

Often, dietary guidance emphasizes the importance of eating whole fruit and vegetables

whilst reducing the consumption of foods high in fat and sodium, i.e., french fries. Indeed

fruits and vegetables are generally have a low energy density but the nutritional

contribution of standard servings of fruits and vegetables varies widely (Hornick &

Weiss,2011). Moreover, the content of phytochemicals also varies greatly and is often not

listed in nutrient databases (Song et al.,2010).

Many constituents of fruits and vegetables (fibre, fat and water soluble vitamins, sterols

and phytochemicals) have also been associated with reduced risk for CVD, which

supports the notion that a high fruit and vegetable consumption might protect against

cardiovascular events via a number of different mechanisms (Knekt et al.,1996; Rimm et

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al.,1996; Liu et al.,2001; Padayatty et al.,2003; Genkinger et al.,2004; Pereira et

al.,2004). These nutrients act through a variety of mechanisms, such as reducing

antioxidant stress, improving the plasma lipoprotein profile, lowering blood pressure,

improving insulin sensitivity, and hemostasis regulation (Van Duyn & Pivonka,2000;

Bazzano et al.,2003, Wannamethee et al.,2006). The benefit of a diet rich in fruits,

vegetables, and grains is attributed to the complex mixture of micronutrients,

phytochemicals, and fiber (Samman et al.,2003). Therefore, the additive and synergistic

effects of these nutrients in fruits and vegetables may be responsible for their potent

antioxidant and anticancer activities (Wolfe et al.,2008, Song et al.,2010). This may

partially explains why no single antioxidant can replace the combination of nutrients in

fruits and vegetables in achieving the observed health benefits.

Overall, several reviews and meta-analyses examining intakes of fruits and vegetables

have reported an inverse association between intake of fruit and vegetables and CVD

occurrence, although epidemiological evidence for causality is controversial (Dauchet et

al.,2005; Dauchet et al.,2006; He et al.,2006; Agudo et al.,2007; He et al.,2007; Wang et

al.,2014). The magnitude of the favorable association remains uncertain because of

differences in methodological approaches, analytical techniques, and the definition of

outcomes.

In this article, we will review the evidence concerning the link between fruits and

vegetables consumption and the risk of CVD.

Health Effects of Fruits and Vegetables

The protective effects of fruits, vegetables, and whole grains against several chronic

diseases are well-documented, however, the evidence is linked to whole foods, rather

than supplements of individual dietary constituents, hence the actions of individual

dietary constituents do not fully explain the observed health benefits of diets rich in fruits

and vegetables. Supplementation with individual antioxidants has been investigated in

randomized clinical intervention trials, but have not consistently shown a benefit

(Ommen et al.,1996, Stephens et al.,1996, Yusuf et al.,2000). Isolated components of the

diet may either lose their bioactivity, may not behave the same way as they do in whole

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foods, or may require other constituents of the whole food for their full functional activity

(Hennekens et al.,1996).

Fruit and vegetables differ in their nutrient content, the manner in which they are

prepared and eaten, and the amounts in which they are eaten daily. Such differences have

important implications for the analysis of fruit and vegetable intake and their

consequential impact on chronic disease risk. Other factors, potentially confounding the

interpretation of fruit and vegetable intake on cardiovascular risk, include the wide

variety of fruit and vegetables consumed globally, each with very different micro- and

macronutrient, contents, and their potential interactions with other dietary components.

These make the interpretation of studies of fruit and vegetable consumption on human

health in prospective cohort studies or randomized controlled trials. Few studies have

investigated the intake of fruits and vegetables separately as in the European Prospective

Investigation into Cancer and Nutrition (EPIC) study (Sargeant et al.,2001). Furthermore

the inclusion of legumes and potatoes in the vegetable category as in the Atherosclerosis

Risk in Communities (ARIC) Study (Steffen et al.,2003) has not been consistently

applied.

The role of fruit or vegetable juices as distinct from that of the whole fruit and vegetable

has not been well studied. Fruit and vegetable juices are obviously different in form from

whole fruit or vegetables, and often lack much of fiber of the latter. In addition, they are

often sweetened with a variety of substances or added sugar, which contribute to their

energy density but not necessarily to their protective function against chronic diseases.

However, the beneficial health effects of consuming fruits are partially attributable to

their antioxidant activity, which is present in fruit juices. For example, the vitamin C

content of citrus juices and polyphenols content of pomegranate juice are considered

protective against oxidative stress and atherogenesis (Aviram et al.,2002, Vinson et

al.,2002). Nevertheless it is worth noting that the antioxidant content of juices, measured

in vitro, does not necessarily translate into bioavailable antioxidant activity, as the

absorption of some antioxidants is poor (Vinson et al.,1995).

Whilst fresh, cooked, and processed fruits and vegetables including frozen and canned,

100% fruit juices, 100% vegetable juices, and dry fruits are all considered as servings of

fruits and vegetables a day, processing of fruit and vegetables alters their structure and

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induces significant changes in chemical composition, nutritional value, and

bioavailability of bioactive compounds. These may all have an impact on their potential

protective effects. Fruit juices, for example, whilst containing less fiber than whole fruit,

may be a good source of phytochemicals (Ruxton et al.,2006). Vegetables are often

cooked before consumption, which causes a significant loss of water-soluble and heat-

sensitive bioactive compounds (Ruiz-Rodriguez et al.,2008). In contrast, processing can

enhance the availability of some bioactive compounds. It has been shown for example

that heat treatment improves the bioavailability of lycopene from tomatoes (Rao &

Agarwal,1999) and carotenoids from carrots (Hornero-Me´ndez & Mı´nguez-

Mosquera,2007). Furthermore, processing could convert folate polyglutamate in

vegetables into monoglutamate, which has better bioavailability (Melse-Boonstra et

al.,2004).

Much attention has been paid to antioxidant vitamins found in fruits and vegetables, yet

these foods are also rich in fiber (Smith & Tucker,2011) and nitrate (Lidder &

Webb,2013) and, therefore, a diet rich in fruits and vegetables will also be rich in a

complex mixture of micronutrients, phytochemicals and fibre, with the exact combination

dependent on the range of fruits and vegetables consumed.

The protective effect of dietary fibre on cardiovascular risk is biologically plausible, and

there are many potential mechanisms through which fibre may act on individual coronary

risk factors. Soluble, viscous fibre types can affect absorption from the small intestine

because of the formation of gels that attenuate postprandial changes in blood glucose and

lipoproteins (Lunn & Buttriss,2007). The formation of gels also slows gastric emptying,

maintaining levels of satiety and contributing towards less weight gain (Lunn &

Buttriss,2007). Soluble fibre and resistant starch molecules are additionally fermented by

bacteria in the large intestine, producing short chain fatty acids, which help reduce

circulating cholesterol levels (Slavin et al.,1999).

It has been suggested that an increased intake of salicylate from foods might have

contributed to the decline in CVD (Ingester & Feinleibl,1997). However, the impact of

salicylate content of foods remains controversial. Salicylates per se do not affect

thromboxane B2 formation or platelet aggregation. It is acetylsalicylate that is able to

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inactivate cyclo-oxygenase and subsequently affect cardiovascular risk (Janssen et

al.,1996).

Other nutrients in fruits and vegetables, for example fructose, are thought to be dietary

risk factors for CVD. Because of its low glycemic index, fructose was suggested as an

alternative sweetener for diabetic patients (Henry et al., 1991). However, in patients with

overt hyperglycemia and insulin deficiency, fructose may be associated with a greater

increase in blood glucose concentrations than in diabetic patients with less elevated blood

glucose concentrations (Swanson et al., 1992). Moreover, fructose induced

hypertriglyceridemia in non-diabetic subjects susceptible to elevated serum triglycerides

has also been a matter of concern (Henry et al., 1991). This is particularly relevant in

patients with diabetes, among whom hypertriglyceridemia is the most common lipid

abnormality and may increase the risk of CVD.

Bioactive Components of Fruits and Vegetables

A number of mechanistic hypotheses involving specific constituents of fruits and

vegetables have been proposed. The antioxidant hypothesis suggests antioxidant vitamins

such as C and E, β-carotene and other carotenoids; antioxidant minerals such as selenium

and zinc and other antioxidant compounds such as flavonoids are protective

(Witztum,1994). The lipid oxidation hypothesis, proposes that LDL cholesterol is

modified to oxidized LDL by free radicals, and this is a potently pro-atherogenic

lipoprotein particle (Witztum & Berliner 1998). When circulating LDL molecules are

present at high levels in blood, they infiltrate the artery wall and increase intimal LDL,

which can then be oxidized by free radicals. This oxidized LDL is more atherogenic than

native LDL and serves as a chemotactic factor in the recruitment of circulating

monocytes and macrophages. Since oxidized LDL plays a key role in the initiation and

progression of atherosclerosis, giving dietary items with antioxidant activity capable of

preventing LDL oxidation has been a putative therapeutic approach. Dietary

micronutrients provided as antioxidants may be incorporated into LDL and may then be

preferentially oxidized when the LDL is exposed to free radicals; this may occur before

any extensive oxidation of the sterol or polyunsaturated fatty acids can occur (Sanchez-

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Moreno et al., 2000). Therefore, dietary micronutrients might retard the progression of

atherosclerotic lesions by reducing the formation of oxidized LDL.

Inflammation is thought to promote the initiation and progression of atherosclerosis, and

is also involved in the acute thrombotic complications of atherosclerosis (Libby et al.,

2002). The relationship between high-sensitivity C-reactive protein (hs-CRP), a marker

of systemic inflammation, and the total antioxidant capacity (TAC) of the diet has

reported that dietary TAC is independently and inversely correlated with hs-CRP

(Brighenti et al.,2005). The authors suggest that this could be one of the mechanisms

behind the protective effects of fruits and vegetables.

The relationship between dietary fruit and vegetables and atheroma formation has been

explored in animal models and in vitro cell culture systems. However, the relevance of

these models to human disease remains contentious (Wolfe et al.,2008; Song et al.,2010;

Wang et al.,2014).

The benefits of a diet rich in fruits and vegetables may also be due to the displacement of

other less healthy foods. The presumptive protection afforded by antioxidants present in

fruits and vegetables (Knekt et al., 1994; Kok et al.1987; Losonczy et al.,1996; Yochum

et al.,2000) was originally based on the measurement of several biochemical markers as

an index of fruit and vegetable intake (vitamin E, carotenoids, vitamin C, and

phytochemicals). However, dietary phytochemicals have been shown to have roles in the

regulation of prostaglandin synthesis, reduction of vascular tone, inhibition of platelet

aggregation, regulation of cholesterol synthesis and absorption, and reduction of blood

pressure and these may not be reflected by the measurements of the biomarkers that have

been used previously.

Of the many nutrients present in fruit and vegetables; dietary vitamins have been

evaluated most extensively in literature. Despite the abundance of observational

epidemiological data supporting their role in CVD prevention (Marchioli et al.,2001;

Knekt et al.,2004; Ye & Song,2008), intervention trials with folate, carotenoids, and

vitamin C have failed to demonstrate convincingly that these vitamins have a role in

preventing CVD and cardiovascular mortality (Vivekananthan et al.,2003; Cook et

al.,2007; Albert et al.,2008; Sesso et al.,2008). In this context, the U.S. Preventive

Services Task Force concluded that the current evidence is insufficient to assess the

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balance of benefits and harms of the use of single- or paired-nutrient supplements (except

β-carotene and vitamin E that have proven to be harmful) for the prevention of CVD and

cancer (Moyer, 2014).

Moreover, a meta-analysis evaluating the effects of combinations of multivitamin and

mineral supplements on prevention of CVD found no evidence of a protective effect

(Huang et al.,2006). Factors that might have affected the results include inappropriate

dosing, insufficient duration of intake, or the stage of disease at which the intervention

was introduced. A further possibility is that CVD prevention trials might have recruited

participants not likely to benefit from treatment, such as those with a satisfactory vitamin

status. In order to translate the knowledge of molecular and cellular mechanisms of

atherosclerosis more effectively into clinical benefit, the methodological issues of the

current clinical trials must be interpreted with caution.

The tendency of researchers to pursue hypotheses based on single nutrients, may

underestimate the potential benefits of chemically complex as foods. The outcomes of the

β-carotene and α-tocopherol trials may be particularly instructive in this regard

(Greenberg & Sporn,1996, Rowe,1996).

Observational Studies

Table (1) shows a summary of observational studies on the relationship between fruit and

vegetable consumption and prevention of CVD, which had ecological, case-control, or

prospective cohort designs. Given the extreme variability of observational studies design,

trend analyses of dose-response relationships do not often reveal statistically significant

associations, as shown in Table (2). These studies have varied considerably in their

inclusion criteria, recruitment methods, sample size, duration of follow-up, definition of

events and adjustment factors, and presentation of results. More attention will be required

on the design of appropriate long-term prospective studies because they offer the most

useful methodological compromise for studying associations between nutritional factors

and CVD, particularly for primary prevention.

Methodological differences among observational studies include dietary assessment

methods, the variety of fruit or vegetables being investigated, the definition of the

reference group, and the choice of exposure categories. Measurement error in dietary

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assessment is a persistent problem when assessing associations between diet and diseases

(Horner et al., 2002). The bias can be large and in either direction. In addition, where

errors exist in the measurement of confounders, the bias would work in the direction of

confounding, which may either exaggerate or dilute the estimated association

(Willett,1990). Other potential biases cannot be ruled out, such as those due to

misclassification of fruit and vegetable intake by the dietary assessment method, the

number of groups of fruit and vegetable consumed, and various reference categories

between individual studies (Lee and Niemen, 2003). Findings from validation studies

using biomarkers suggest that measurement error in dietary exposures will often result in

attenuated estimates, which may often be quite severe (Willett,1990).

Therefore, more objective and accurate indices of fruit and vegetable intake, such as

nutritional biomarkers, are becoming popular. Such biomarkers of intake need to be able

to discriminate between differences in intakes should be non-invasive or minimally

invasive, reproducible, easily measured, and highly responsive to the intervention being

carried out (Crews et al., 2001).

Data from dietary assessments in prospective studies were often unadjusted for total

calorie intake and only assessed at a single time-point (baseline). Moreover, the reliability

and accuracy of these methods to assess long-term nutritional habits are limited and have

not always been robustly validated. Therefore, heterogeneity complicates estimation of

the magnitude of effects, and the comparison of results between studies (Dauchet et al.,

2009).

It is possible that variation between study populations on the types of fruits and

vegetables and whether cooked or raw vegetables are most commonly consumed may

introduce heterogeneity. Sometimes heterogeneity is introduced by including vegetables

that contain more protective nutrients, which yield a stronger association in comparison

with weaker association resulting from including heavily processed vegetables. Stratified

analysis for fruits and vegetables separately may reduce the heterogeneity. Moreover,

adjustment for major confounding factors in the studies that have been included should

reduce the potential bias due to these other dietary and lifestyle factors.

Limitations of observational studies include the problem of residual confounding, which

also remains when meta-analyses of these studies are undertaken. Exploration of

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confounding factors, through meta-regression, has not yet confirmed that adjustment for

such confounders sufficiently explained the strength or direction of associations observed

(Lawlor et al.,2004).

Risk reduction estimates from cohort studies have generally been less impressive than

initially observed in ecological or case–control studies. Several reasons for this are

possible including an improvement of the dietary habits in the comparator population

over the duration of these studies, which could affect the outcomes of prospective follow-

up. Also, the lack of comparability of dietary assessment methods combined with

inaccurate measurements makes it difficult to detect associations or weaken these

associations. Indeed meta-analyses of observational studies report relative risks indicative

of a protective effect of fruit and vegetables, but they also show substantial inter-study

heterogeneity, which might be explained by methodological issues or publication bias

(Dauchet et al.,2006, He et al.,2007).

The observation of a favorable association between fruit and vegetable consumption and

cardiovascular risk does not necessarily indicate causality; hence, fruit and vegetable

intake might simply indicate a reduced intake of unhealthy nutrients, such as saturated

fats and salt. Furthermore, social and behavioral factors may often be associated with a

health-conscious lifestyle, and the impact of these cannot always be measured precisely

or might not be appropriately considered in statistical analyses. Therefore, the

confounding effects in observational studies might not be fully appreciated.

Interventional Studies

Because observational studies do not control for unmeasured confounders, the causal

relationship remains to be established using randomized controlled trials. Although

nutritional prevention trials provide the most robust estimate of causal effects, they also

have some limitations that may make their interpretation difficult. The success of these

studies relies on compliance of the participants for the duration of the study, which can be

over several years. Obviously, a double-blind controlled trial is not feasible in the context

of interventions with food items, leaving the possibility for biases in the assessment of

end-points. Secondly, increased consumption of fruit and vegetables might induce

changes in other components of the diet. For instance, in the DASH trial, fruit and

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vegetables were substituted by snacks to achieve equivalence of energy intake in the

control and experimental diet. Thirdly, results of tightly controlled intervention trials

cannot easily be extrapolated to a free-living general population, or to public health

policies. Finally, the difficulty of achieving a substantial modification of diet in long-term

studies hampers the interpretation of the results.

Not surprisingly, given their methodological difficulties, few randomized nutritional

trials have been conducted to evaluate the effect of high fruit and vegetable intake on the

incidence or recurrence of CVD (reviewed by Brunner et al.,2006). It must also be

emphasized that changes in fruit and vegetable intake achieved in these studies have been

generally modest in these clinical studies.

In the Indian heart study, a semi-vegetarian diet enriched with fruits, vegetables, whole

grains, and nuts reduced coronary death and nonfatal myocardial infarction (Singh et

al.,1992). A vegetarian diet is associated with a lower blood cholesterol concentration, a

lower incidence of stroke, and a lower risk of mortality from stroke or ischemic heart

disease (Bazzano et al.,2002, Bazzano et al.,2003) when compared to a mixed diet.

However, vegetarians often have a higher consumption of whole grains, soy, and nuts, all

of which have significant cardio-protective effects (Kelly & Sabate,2006, Mellen et

al.,2008).

In the Diet and Reinfarction Trial (DART) II, recommendations aimed at promoting

increased consumption of fruit, vegetables, fiber, and orange juice had no effect on

cardiac death in patients with angina pectoris (Burr et al.,2003). However, the small

difference in fruit and vegetable intake between study and control participants (~20 g per

day) may have contributed to the lack of significant benefit (Ness et al.,2004, Burr,

2007).

In the Women’s Health Initiative (WHI) Randomized Controlled Dietary Modification

Trial, fruit and vegetable intake in the group that received nutritional advice had

increased only slightly (by 1.2 servings per day), compared with the control group and

the two groups did not differ significantly in the rate of major CVD events (Howard et

al.,2006a).

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The Lyon study showed major benefits of a Mediterranean Diet but given the complexity

of this diet and the use of margarines rich in α-linolenic fatty acid, these benefits cannot

be attributed to fruit and vegetable consumption alone (de Lorgeril et al.,1999).

Similarly, the Prevencio´n con Dieta Mediterra´nea (PREDIMED) study has

demonstrated that adherence to the Mediterranean Diet is associated with an increase in

serum markers of atheroma plaque stability which may partly explain the cardio-

protective role of Mediterranean Diet (Casas et al.,2014).

Nevertheless, when the short-term effects of Mediterranean diets versus those of a low-

fat diet were investigated on intermediate markers of cardiovascular risk, significant

dietary changes were only seen in olive oil, nuts and red meat within the intervention

groups (Estruch et al.,2006). Therefore, any clinical benefit of the Mediterranean Diet

cannot be attributed to a change in fruits and vegetables intake.

Fruits and Vegetables and Cardiovascular Risk Factors

Investigating the effects of fruit and vegetable intake on classical cardiovascular risk

factors is crucial in determining biological plausibility of a causal relationship between

fruit and vegetable consumption and CVD.

Hypertension

Fruit and vegetables are good sources of magnesium and potassium, which have been

inversely associated with hypertension in previous studies (Zhang et al.,2012). The

Dietary approaches to stop Hypertension (DASH) study showed that fruit and vegetable

consumption was associated with a significant decrease in blood pressure, in which the

effect was more pronounced in hypertensive patients than in normotensive participants

(Broekmans et al.,2001, Sacks et al.,2001). This might be attributed to an increase in the

intake of potassium, magnesium, or some other component in fruits and vegetables, or

alternatively a reduced intake of sodium (Hord et al., 2009). A number of intervention

studies have given conflicting results on the beneficiary effect of increased fruits and

vegetables intake on blood pressure (Appel et al.,1997; McCall et al.,2009; Berry et

al.,2010). This is a consequence of the potential differences in study designs, participant

15

characteristics (in terms of degree of hypertension at baseline) and methods used to

achieve an increase in fruits and vegetables intake.

Results from a large randomized controlled trial, to increase daily consumption of fruit

and vegetables to at least five portions among healthy study participants randomly

selected from primary health care centers, have shown that increased consumption of

fruit and vegetables can contribute to a small decrease in blood pressure (John et

al.,2002).

Hyperlipidemia

Evidence for the effects of increased fruits and vegetables intake on

hypercholesterolaemia is limited to date, with many studies either not being specifically

designed to test the effects on hypercholesterolaemia, or where increased fruits and

vegetables was combined with some other dietary intervention, such as reduced fat

intake. The large randomized controlled trial, which demonstrated a positive effect of

increased fruits and vegetables intake on blood pressure, showed no effect on blood

levels of total cholesterol (John et al.,2002).

Several components of fruit and vegetables have cholesterol-lowering properties,

particularly dietary fiber (Satija & Hu,2012). Similarly, ingestion of large quantities of

phytosterols (>2 g per day) lowers plasma LDL-cholesterol levels (Thompson &

Grundy,2005). Since the available evidence demonstrates that viscous fibres, plant

sterols, vegetable proteins and nuts can each independently lower serum cholesterol by

5–10% (Anderson et al.,2000). The Portfolio Diet was introduced as a dietary approach

based on current dietary recommendations that is feasible, accessible, and has maximized

the metabolic advantages of plant-based foods. It is in compliance with the dietary

recommendations of the Adult Treatment Panel III of the National Cholesterol Education

Program. The major recommendations are that <7% energy should come from saturated

fat, dietary cholesterol intake should be <200 mg/d, dietary fibre intake should be 20–30

g/d, and 50–60% energy should come from carbohydrate, 15% energy from protein and

25–35% energy from fat (NCEP, 2001). It is assumed that the effect of combining these

components may be additive and as such leads to clinically-significant reductions in

16

serum cholesterol and consequently lower the risk of developing CVD (Jenkins et

al.,2002).

Diabetes

The relationship between fruit and vegetable intake and the risk of type II diabetes has

not been established and cohort studies have produced conflicting results (Bazzano et

al.,2008, Villegas et al.,2008). The discrepancies between studies could be explained by

confounding factors and by the possible antagonistic effects of the sugars, fiber, and

antioxidants in fruit and vegetables. The results from a few prospective studies regarding

fruit and vegetable intake and risk of type 2 diabetes mellitus are not entirely consistent.

For instance, an inverse relation between intake of vegetables, but not of fruits, and risk

of type 2 diabetes mellitus was observed in the Nurses’ Health Study (Colditz et

al.,1992). An inverse relation between intake of fruit and vegetables and diabetes mellitus

risk in women, but not for men in the National Health and Nutrition Examination I

Follow-up Study (Ford & Mokdad,2001). The Iowa Women’s Health Study did not show

an association between fruit and vegetable consumption and incident diabetes (Meyer et

al.,2000). Also, none of these studies examined different types of fruits and vegetables in

relation to the risk of type 2 diabetes mellitus.

Compared with carbohydrate-dense foods, fruits and vegetables (with the exception of

potato) generally contribute to a lower glycemic load (a measure of the postprandial

glucose-raising potential of dietary carbohydrates). In prospective studies, a diet low in

glycemic load has been associated with lower risks of type 2 diabetes mellitus and CVD

(Liu et al.,2001, Liu et al.,2000b). The rich fiber content of whole fruits and vegetables

may be partly responsible for this response, but other factors such as the physical

structure of fruits and vegetables, antioxidants, carotenoids, flavonoids, and many

enzyme inhibitors may play important roles as well (Liu et al.,2001, Ylonen et al.,2003).

A link has been proposed between increased consumption of green leafy vegetables and

reduced incidence of diabetes, albeit is based on a limited number of studies (Carter et

al.,2010, Cooper et al.,2012), with other classes of fruits and vegetables, such as root

vegetables, also potentially playing a role.

Randomized prevention trials have demonstrated the effectiveness of complex lifestyle

interventions (including physical activity and body weight reduction) on reducing the risk

17

of diabetes, but the contribution of fruit and vegetables per se in these studies is difficult

to estimate given the complexity of the interventions (Pan et al.,1997, Tuomilehto et

al.,2001).

Obesity

It has been reported that replacing energy-dense foods with fruit and vegetables could

help to decrease calorie intake and, therefore, body weight (Epstein et al.,2001).

Accumulating evidence indicates that the combination of increased fruit and vegetable

intake, together with other dietary recommendations, might promote satiety and weight

loss in overweight individuals (Tohill,2005). Although the effect of fruit and vegetable

consumption on weight control has been scarcely investigated in clinical settings, it has

been found that decreased fat intake combined with increased fruit and vegetable intake

results in greater weight loss when compared with low-fat diets alone (Howard, et

al.,2006b).

Multiple pathways are expected to be involved, high water and fiber content and low-

energy density of whole fruits and vegetables may promote weight control by reducing

total energy intake through increasing satiety or by substituting for foods with high-

energy densities (Pereira & Ludwig, 2001). Multiple enzymatic inhibitors in fruits and

vegetables may also affect body weight through other pathways, including the

modulation of metabolic efficiency and/or insulin secretion or action (Liu et al.,2003).

Epidemiological Studies and their Limitations

There is still considerable scientific uncertainty about the relationship between specific

dietary components and cardiovascular risk. Reasons for this uncertainty include the high

doses of nutrients used in intervention trials that may have resulted in unexpected effects

including pro-oxidant activity of excessive doses of a single antioxidant, and residual

confounding in cohort studies due to imperfect classification of factors such as smoking

that may have biased the results. Since the effect of antioxidants on mortality may vary

by oxidative burden, associations between fruits and vegetables and CVD may be

affected by cigarette smoking and body mass index, both sources of oxidative stress.

Interactions between the numerous nutrients found in fruit and vegetables are difficult to

evaluate in biological experiments. Moreover, different varieties of fruit and vegetables,

18

growing techniques, industrial processing, and storage and cooking methods can strongly

affect the composition and properties of the nutrients. Perhaps the most effective

combination of vitamins or the optimal biochemical structure for synthetic molecules

used for CVD prevention have not been identified.

Responsiveness of biomarkers may be altered by lifestyle and other factors, such as age,

smoking behavior, physical activity, and the presence of low-grade inflammation. Such

factors may have affected the response of biomarkers in intervention studies, but the

studies are too heterogeneous in terms of design, level of control of overall dietary intake,

duration, and the nature of the control group to allow exploration of these issues. The

level of nutrients in food will directly affect biomarker response and may vary by

individual variety of fruit or vegetable (Kurilich & Juvik, 1999), production conditions

(Kopsell and Lefsrud, 2006), and processing, storage, and cooking methods, particularly

for vitamin C (Castenmiller et al., 1999).

The current literature does not allow us to examine the association of cardiovascular risk

with specific fruits and vegetables or with fruit and vegetable groups, or to examine the

effect of seasonal deficits in intake; nor could we examine the effects of secular changes

in diet. International collaboration for food based analyses of existing cohort studies,

where the data is available, could help refine the current state of knowledge. It will be

important to address this in future studies to provide clarification regarding the optimum

protective level of fruits and vegetables intake, and therefore be as directly relevant to

dietary recommendations and policy as possible.

Future Prospects

Because of the physical and biochemical complexity of whole foods, much uncertainty

remains regarding the direct relation between fruit and vegetable intake and CVD risk. As

fruits and vegetables are a complex food group, judging biological plausibility of the

association demonstrated in observational studies is extremely challenging, so is defining

their exact protective mechanism in interventional trials.

Results from the WHO MONICA study show that inter-population differences are very

inadequately explained by the classical cardiovascular risk factors, suggesting that the

search for new determinants of vascular risk should be explored further (WHO MONICA

19

project, 1994). A recent randomized clinical trial shows that fruit and vegetable intake in

hypertensive patients resulted in a dose-dependent improvement in endothelium

dependent forearm blood flow (McCall et al.,2009). Nevertheless, this is in contrast to

other negative ascorbic acid supplementation studies (Chen et al.,2006). Thus, increasing

fruit and vegetable consumption is likely to have beneficial effects due to synergistic

effects of bioactive compounds that improve the vascular phenotype but may not be

readily detected by routine clinical or biochemical examination. Endothelial function is

known to be a surrogate measure of vascular function with known diagnostic and

prognostic values.

As we continue to identify the roles of nutrients in foods in the complex pathways that

contribute to CVD risk and/or prevention, scientists are focusing on genetic modulation

of these pathways so that better guidance may be developed for subsets of the population

at differential risk. Thus, the use of biological and genetic markers in future studies may

help to clarify whether a causal relationship exists between fruit and vegetable intake and

CVD risk and/or prevention in a subset of the population in whom interventions would be

best targeted. It has recently been shown that there may be genetic variation in the

metabolic response to intake of compounds found in fruits and vegetables, such as

vitamin C (Timpson et al.,2010) and carotenoids (Borel,2012).

A recent human nutrigenomics study also confirmed the importance of whole food

instead of a single phytochemical (Milenkovich et al.,2011). The effects of orange juice

and pure citric phytochemical hesperidin were investigated on the expression of genes in

leukocytes in healthy volunteers after consumption of orange juice, hesperidin, or

placebo for 4 weeks. Global gene expression profiles were determined using human

whole genome cDNA microarrays. Both orange juice and hesperidin consumption

significantly affected leukocyte gene expression. Orange juice consumption induced

changes in the expression of 3422 genes, hesperidin intake modulated the expression of

1819 genes, and 1582 genes were in common in both groups. Many of the protein

products of these affected genes were involved in chemotaxis, adhesion, infiltration, and

lipid transport, suggesting lower recruitment and infiltration of circulating cells to

vascular wall, lower lipid accumulation, and formation of the atherosclerotic plaque.

20

Conclusions

Many cardiovascular risk factors, including hyperlipidemia, hypertension, obesity and

diabetes are substantially influenced by dietary factors. There is good evidence that a

high consumption of vegetables and fruit is associated with a reduced levels of

adiposity and consequently a lower risk of hypertension, and type 2 diabetes

mellitus. Understanding the relation between fruit and vegetable consumption and CVD

is important for guiding consumer education and informing dietary guidelines to reduce

cardiovascular risk. The key is to increase the total up to above five daily servings of

fruits and vegetables in all forms. Fruit and vegetables should be eaten as part of a

balanced diet, as a source of vitamins, fiber, phytochemicals, and water. People should

obtain bioactive micronutrients from a wide variety of whole foods for optimal nutrition

and health well-being, not from dietary supplements, or from foods enriched in cereal or

vegetable derived fiber.

Early efforts focused on identifying protective nutrients, for example vitamin E, vitamin

C, and β-carotene, have proven to be disappointing when tested in clinical trials. This is

almost certainly owing to the complexity of fruit or vegetables and the large number of

bioactive compounds present, and also because of other dietary sources containing these

bioactive compounds. Rather, the evidence now suggests that a complicated set of many

nutrients interact to influence CVD risk. Therefore, it is important to focus on whole

foods and dietary patterns to impact on CVD risk reduction.

Evidence that fruit and vegetable consumption has a protective effect against CVD comes

mainly from observational studies. Uncertainty remains about the real magnitude of the

relationship between fruit and vegetable intake and the occurrence of CVD. Results from

randomized controlled trials do not show conclusively that fruit and vegetable intake

protects against CVD, in part because the dietary interventions have not been intensive

enough to enable optimal analysis of their putative effects.

Protective mechanisms may involve not only known bioactive nutrients such as fiber and

potassium, but also functional aspects of fruit and vegetable intake such as low glycemic

load and energy density. Taken together, the totality of the evidence accumulated so far

does appear to support the notion that increased intake of fruits and vegetables may

reduce cardiovascular risk.

21

The effect of specific classes of fruits and vegetables combined, fruit or vegetables alone,

the effect of quantity versus variety of fruits and vegetables consumed and the degree of

processing on demonstrated associations remains uncertain. Some of these uncertainties

could be answered in future pooling projects.

22

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33

Table (1): Summary of prospective studies on the association between fruits and

vegetables consumption and CVD

Study Study cohort Exposure

measure

Follow-

up

period

Main findings Reference

The Adventist

Health Study

Prospective

cohort

investigation of

31,208 non-

Hispanic white

California

Seventh-Day

Adventists

65 item

FFQ

6years No associations between

CVD risk with fruit index

or legumes

Fraser et

al.,1992

The Caerphilly

Study

Prospective

Ischaemic Heart

Disease Study of

a sample of 2512

men aged 45-59

years

FFQ 5years There was a trend of

increasing IHD risk with

decreasing vitamin C

intake, the relative odds of

an IHD event being 1.6

among men in the lowest

one-fifth of the vitamin C

distribution, but this was

not statistically significant

Fehily et

al.,1993

Health

Professional

Follow Up

Study

39,910 U.S. male

health

professionals 40

to 75 years of age

131 item

FFQ

4years Evidence of an association

between a high intake of

vitamin E and a lower risk

of coronary heart disease

in men

Rimm et

al.,1993

The Zutphen

Elderly Study

805 men aged 65-

84 years

cross-check

dietary

history

5years Flavonoids in regularly

consumed foods may

reduce the risk of death

from CHD in elderly

men

Intakes of tea, onions,

and apples were also

inversely related to

coronary heart disease

mortality, but these

associations were

weaker

Hertog et

al.,1993

Longitudinal

population study

in Finland

A cohort of 5,133

Finnish men and

women aged 30-

69 years and free

from heart

disease at

baseline

dietary

history

14years Antioxidant vitamins

protect against CHD, but it

could not exclude

possibility that foods rich

in these micronutrients

also contain other

constituents that provide

the protection

Knekt et

al.,1994

The

Massachusetts

1299 elderly

living in

43category

FFQ

4.75year Increased dietary intake of

carotenoids decreases the

Gaziano et

al.,1995

34

Health Care

Panel Study

Massachusetts

residents

risks of CVD mortality;

however, confounding

cannot be ruled out

The Department

of Health and

Social Security

nutritional

survey

Elderly people

living in the

community who

had taken part in

8 areas in Britain

7day

weighed

record

20year Vitamin C concentration

in elderly people is

strongly related to

subsequent risk of death

from stroke but not from

CHD

Gale et

al.,1995

Framingham

Study

832 men, aged 45

through 65 years,

who were free of

cardiovascular

disease at

baseline

24hr recall 20year Intake of fruits and

vegetables may protect

against development of

stroke in men

Gillman et

al.,1995

The Western

Electric study

1,556 employed,

middle-aged

men, aged 40-55

years

Diet

history

24year Consumption of foods rich

in vitamin C and beta-

carotene reduces risk of

death in middle-aged men

Pandey et

al.,1995

Postmenopausal

women

34,486

postmenopausal

women with no

cardiovascular

disease

127item

FFQ

7year In postmenopausal

women the intake of

vitamin E from food is

inversely associated

with CHD mortality

and that such women

can lower their risk

without using vitamin

supplements

The intake of vitamins

A and C was not

associated with lower

risks of CHD mortality

Kushi et

al.,1996

Longitudinal

population study

in Finland

A cohort of 5,133

Finnish men and

women aged 30-

69 years and free

from heart

disease at

baseline

dietary

history

14years People with very low

intakes of flavonoids have

higher risks of CHD

Knekt et

al.,1996

Prospective

observation of

vegetarians,

semi-

vegetarians, and

meat eaters in

UK

10,802 men and

women in the UK

aged between 16

and 79

13.3year Deleterious effects of

saturated animal fat and

dietary cholesterol are

more important in the

aetiology of IHD than

the protective effect of

dietary fibre.

Reduced intakes of

saturated animal fat and

cholesterol may explain

lower rates of IHD

Mann et

al.,1997

35

among vegetarians

compared with meat

eaters

The Women’s

Health Study

39876 female

health

professionals

with no previous

history of CVD

or cancer

FFQ 5year Higher intake of fruit and

vegetables may be

protective against CVD

and support current dietary

guidelines to increase fruit

and vegetable intake.

Liu et

al.,2000a

A combined

analysis of the

Nurses’ Health

Study and the

Health

Professionals’

Follow-up Study

Nurses' Health

Study: 84 251

women 34 to

59 years of

age. All were

free of

diagnosed

CVD, cancer,

and diabetes at

baseline.

Health

Professionals'

Follow-Up

Study: 42 148

men 40 to 75

years. All

were free of

diagnosed

CVD, cancer,

and diabetes at

baseline.

FFQ

FFQ

NHS:

14year

HPFS:

8year

Consumption of fruits and

vegetables, particularly

green leafy vegetables and

vitamin C-rich fruits and

vegetables, appears to

have a protective effect

against CHD

Joshipura

et al.,2001

The Physicians’

Health Study

22 071 US male

physicians aged

40-84 years

FFQ 12year An inverse association

between vegetable intake

and risk of CHD

Liu et

al.,2001

Alpha-

Tocopherol,

Beta-Carotene

Cancer

Prevention

(ATBC) Study

in Finland

27,110 male

smokers, aged

50-69 years,

without history of

cancer

FFQ 6.1-year Intake of flavonols and

flavones seemed to be

inversely associated with

the risk of lung cancer, but

not with that of other

cancers

Hirvonen

et al.,2001

The first

national health

and nutrition

examination

survey

(NHANES-I)

follow-up study

9608 adults aged

25-74 years

participating free

of CVD at the

time of their

baseline

FFQ 19year An inverse association of

fruit and vegetable intake

with the risk of CVD and

all-cause mortality in the

general US population

Bazzano et

al.,2002

The

atherosclerosis

risk in

15,792 adults

aged 45-64 years

participating free

FFQ 11year A beneficial effect of

whole-grain and fruit and

vegetable consumption on

Steffen et

al.,2003

36

communities

(ARIC) study

of CAD at the

time of their

baseline

the risks of total mortality

and incident CAD was

suggested but not on the

risk of ischemic stroke

The Prospective

Epidemiological

Study of

Myocardial

Infarction

(PRIME) study

men aged 50-59

years, free of

CHD, who were

recruited in

France (n 5982)

and Northern

Ireland (n 2105)

FFQ 5year Frequency of citrus fruit,

but not other fruits, intake

is associated with lower

rates of acute coronary

events in both France and

Northern Ireland,

suggesting that

geographical or related

factors might affect the

relationship between fruit

consumption and CHD

risk

Dauchet et

al.,2004

The Odyssey

Cohort

8,394 residents in

Washington

County,

Maryland

61item

FFQ

15year Participants with the

highest quintile of fruit

and vegetable intake had a

lower risk of all-cause

mortality and CVD

mortality than those in the

lowest quintile

Genkinger

et al.,2004

A combined

analysis of the

Nurses’ Health

Study and the

Health

Professionals’

Follow-up Study

Over 100000

participants in the

Nurses’ Health

Study and the

Health

Professionals’

Follow-up Study

FFQ Fruit and vegetable intake

was inversely associated

with risk of CVD, with

relative risk for an

increment of 5 servings

daily of 0.88 (95% CI =

0.81 to 0.95), and green

leafy vegetables showed

the strongest inverse

association

Hung et

al.,2004

The Baltimore

longitudinal

study of aging

501 healthy men

at baseline

7day diet

records

18year Protective effects of

low saturated fat and

high fruits and

vegetables intake

against CHD mortality.

The combination of

both behaviors is more

protective than either

alone, suggesting that

their beneficial effects

are mediated by

different mechanisms.

Tucker et

al.,2005

Nurses’ Health

Study

72 113 women

who were free of

CVD and cancer

at baseline

FFQ 18years A more prudent diet

containing high

intakes of fruits and

vegetables was associated

with a reduced risk of

Heidemann

et al.,2008

37

cardiovascular mortality

and a reduced risk of all-

cause mortality when

comparing individuals in

the highest to the lowest

quintile of diet prudency.

The Jichi

Medical School

cohort study

10,623

participants

(4147 men, 6476

women) who had

no history of

CVD or

carcinoma at

baseline

5 category

FFQ

10.7

years

Frequent intake of citrus

fruit may reduce the

incidence of CVD,

especially cerebral

infarction, in men and

women

Yamada et

al.,2011

CAD: coronary artery disease, CHD: coronary heart disease, IHD: ischemic heart disease, CVD: cardiovascular disease, FFQ: food

frequency questionnaire

38

Table (2): Shortcomings of epidemiological studies

Study design Description Limitation Ecological

studies

To explore relationships between

environmental factors and diseases

at a population level

Do not provide precise information about

individuals in a population

Subject to many uncontrolled biases

Case–control

studies

Individuals diagnosed with disease

are compared with disease-free

individuals

Rely on retrospective assessment of

dietary intake by questionnaire

Subject to selection and recall bias and

potential bias due to the changes in diet

and lifestyle following CVD events Cohort studies

The exposure of people who are

initially assumed to be healthy are

assessed and the group is followed

over a period of time. During the

follow-up period, some individuals

will develop disease

Measurements are made at baseline before

disease is diagnosed, but diet may change

with time

Cohort studies are less subject to recall

bias

Randomized

controlled trials

The participants are allocated to

treatment/ intervention or control/

placebo groups using a random

mechanism. Best for studying the

effect of an intervention

Expensive in terms of time and money

Often short duration with less power to

detect clinical outcomes

Subject to volunteer bias

Ethically problematic